مجله علوم و مهندسی خوردگی
سال سوم شماره 3 (پیاپی 9، پاییز 1392)

The protective mechanisms of zinc-rich coatings include cathodic protection driven by the more active Zn particles and barrier-type protections due to the formation of zinc oxide and sealling the pores between the zinc particles are discussed. Methods of evaluating the performance of these coatings are also discussed and electrochemical evaluation as one of the most important method that can provide quantitative results has been more studied. At the end, two ideas of research performed to improve the electrical conductivity of the coatings to increase the longevity of cathodic protection and improved barrier property to prevent penetration of corrosive agents and slow down intake of zinc metal powder is provided.

In present work, highly homogeneous Ni–P/SiO2 nanocomposite coatings were prepared by a new process on steel substrate. This method was developed by inserting of a sol containing SiO2 to Ni-P electroless bath in the presence of surfactant to produce a composite coating containing well dispersed SiO nanoparticles. The morphology and elemental composition were analyzed using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS) that confirmed the presence of silica in the coating matrix.The Results showed co-deposited SiO2 particles approximately 1.58wt% with no agglomeration. X-ray mapping images of Ni-P/SiO2 showed that SiO2 particles were uniformly distributed in the deposit. Electrochemical impedance spectroscopy and polarization tests showed that addition of nano-SiO2 particles via sol solution increased corrosion resistance of Ni–P coatings in 3.5% NaCl aqueous solution from 2726 Ω to 6985 Ω.

Platinum anodes which are manufactured by electroplating process, are widely used in chlor-alkali industries due to their electrocatalytic properties. Their corrosion resistance, and subsequently their life time is an important parameter because of their high price. In this research, the samples of platinum coating on niobium were electroplated in current densities of 3, 5 and 8 mA.cm-2. After electroplating, Accelerated life test (ALT) on the samples was conducted in a solution composed of sodium chlorate for time durations of 1, 10, 30 and 60 hours. Then electrochemical impedance spectroscopy (EIS) studies were conducted in the solution mentioned for ALT in frequency range of 100 kHz to 0.01 Hz and voltage amplitude of ±10 mV. The results showed that the electroplated sample in current density of 5 mA.cm-2 has the best corrosion resistance after 60 hours of ALT in which the passive layer of Nb2O5 has the least thickness and more platinum is exposed to the electrolyte due to its high corrosion resistance.

Surface mechanical attrition treatment (SMAT) done on MgAZ31B alloy as the pretreatment process for subsequent micro arc oxidation (MAO) coating on titania nanoparticle suspension. severe plastic deformation creates nanostructured surface that effective on subsequent oxidation. In order to compared nanocrystaline and coarse-grained surface, weight, morphology and potentiodynamic polarization test were examined. Surface morphology was analyzed by scanning electron microscopy. The corrosion resistance of coated samples in a simulated body solution (Ringer's solution) was evaluated. The results showed that the surface mechanical attrition treatment not only changed the porosity and surface morphology of coating but also increased corrosion resistance of MgAZ31B alloy.

Zn-SiC nanocomposite coatings were prepared from a sulfate bath solution by pulse electrochemical deposition. In this method, frequency and duty cycle parameters were investigated. Morphological and corrosion properties of these coatings were investigated by FE-SEM and potentiodynamic polarization in 1M NaCl solution. In case of frequency it was observed that by increasing its value from 10 to 100 Hz, the amount of codeposited nanoparticles firstly decreases and then increases in the coatings. By increasing duty cycle from 10 to 75%, the amount of codeposited nanoparticles firstly increases until 50% duty cycle and then decreases in the coatings. Also by increasing the maximum current density, a maximum amount of incorporated nanoparticles is observed. These changes are function of Ton and Toff in pulse current.

Microbiologically influenced corrosion is a major cause of industrial system failures. In the present study, for investigation of microbiologically influenced corrosion in fire-fighting system of Fajr petrochemical company, a variety of methods, including microbial cultivation, X-Ray diffraction, corrosion morphology and inspection of selected points of lines were employed. Different groups of corrosion-causing bacteria were observed in fire-fighting system. Sulfate reducing bacterial count in different parts of the system such as outlet of raw water tank and one hydrant were 103 and 104 per ml, respectively. Also producing bacterial count in some system parts such as outlet of raw water tank was 104 per ml. Heterotrophic bacteria were distributed throughout the system and their count in one hydrant reached 4 × 105 per ml. Furthermore, different groups of bacteria were observed in corrosion product deposits. Sulfate reducing and heterotrophic bacterial count were 104 and 5 × 105 per gram of deposits, respectively. Iron bacteria were also detected in corrosion products. Additionally, analysis of corrosion product deposits using XRD technique demonstrated the presence of Fe2O3, FeO(OH) and FeS. In addition, a variety of pit structures with different shapes, particularly pear-shaped pit, were observed. Pipelines inspection obviously proved formation of discrete deposits, including nodules and tubercles. These results indicated that microbiologically influenced corrosion occurred in fire-fighting system of this industrial unit. Thus, monitoring and mitigation of microbiologically influenced corrosion in fire-fighting systems must be fully considered.

In this study, the degree of sensitization was examined in AISI 304 and 321 type steels by DLEPR method whose experiments have been carried out on steels to examine and determine effect of NaCl, solution motion, pH and O2 on intergranular corrosion. It is seen that as NaCl molarity is increased, both activation and reactivation currents increase and this cause to increase on susceptibility to intergranular corrosion of both steels. In the absence of O2, the activation and reactivation values and degree of sensitization decrease. In addition, low speed of solution motion cause to decrease degree of sensitization of 304 steel. Increasing speed of solution motion will increase the degree of sensitization of 304 steel again. Increasing speed of the solution motion cause to decrease on degree of sensitization in all of the speeds. The increasing concentration of H2SO4 and decreasing pH cause to increase Ia, Ir and intergranular corrosion resistance of 321 steel. As decrease pH, at first the intergranular corrosion resistance of 304 has been increased and afterward decreased.